I agree speaking with management is a good idea and appreciate that perspective. However, there are restaurants here that I know people have either 1)spoken to the staff or 2)read a negative review of their restaurant, and they continue to serve mediocre food. And I would appreciate it if I had a venue for determining which of those here fall into the "avoid" and "go" category. And restaurants change-there is one here I used to love, but now would avoid. This may not be the venue for realistic, honest reviews and if that is the case, so be it. Or perhaps I will just avail myself of the private message venue.
I agree. There are restaurants that we all know people have already tried to talk to. The fact that your original post said you were "disappointed" made me think that this wasn't one of them, and that they were not up to par with your expectations.
Personally, I feel that if you made an honest effort to talk to the staff/management, and your complaints did not receive a sincerely apologetic response and/or correction, I think it's okay to come to the forum and post about it.
Let me emphasize that: POST about it, NOT rail about it. I think that it is extremely helpful to have well-written, thought-out posts detailing the issues that you had with food quality, service, etc. That gives people a good frame of reference, and they can decide for themselves whether they want to chance it. I think the biggest problem that a forum can face is people posting when they're still really P.O.'d about what they just experienced.
Coming on here and writing, "Such&Such Restaurant SUCKS!" isn't going to help anybody. Sleep on it, think over what the real problems were, take into account any influencing factors (maybe there's a large spring break group in town so you had to wait a little longer for a table, maybe you were already agitated because of the screaming baby across the aisle from you, etc.) and then calmly and rationally post about your experience.
So long as you're providing facts (i.e., the food was under/over cooked, we were served the wrong meal, the servers were ill-tempered...) and not just flaming an establishment because you didn't get the table you wanted or some other silly nonsense, I see no reason that you should feel guilty in posting. You might even let people know whether you plan to go back and give them another try, because we're all human, and everybody has an off night from time to time; or whether you are going to steer clear of them for good.
Also, keep in mind that everybody's got an opinion. The restaurants you want to avoid may be your neighbor's favorite haunts... just back it up with facts, and you're completely entitled to your opinion.